In the production line from hydrocarbon wells, in particular gas wells, often a multiphase fluid is transported. The fluid typically comprises gas such as natural gas, light liquid such as hydrocarbon oil or condensate, and heavy liquid, such as water (brine). Further the fluid can contain some solids. The flow rate of the produced fluid can vary, as can the phase composition.
Liquids are normally separated from the gas in so-called separator tanks. However, the gas and liquid may have to travel through long pipelines before separation can take place. In that case the liquid can build up in the pipeline to large slugs. Slugs can also result from a so-called pigging operation of the pipeline, in which a pigging device (for example a sphere) is moved through the pipeline, thereby pushing the liquid out. If these slugs are expected to become really large, the separator vessels would become un-practically big. In that case the liquid slug is collected in a slug catcher.
The slug catcher is normally sized so that it is large enough to receive the volume of fluids, in particular liquids, when pigging operations are being performed or have recently been performed.
The slug catcher can in particular be designed to perform a gravity separation or pre-separation of the fluid into gas, light liquid and heavy liquid.
Although generally satisfactory, slug catcher's efficacy in such separation can be compromised when a large surge of fluids is deposited therein. A particular problem arises if due to high water influx, no hydrocarbon stream can be removed from the slug catcher components. This can result in downstream units, such as a three-phase separator, being starved of hydrocarbons which may cause a shut down of the plant and further downstream processing units, e.g. an LNG plant, a Fischer-Tropsch plant or a methanol plant. As the maximum removal rate of water is usually limited, for example depending on the downstream water processing/treating capacity, it may take a long time before the interface level has dropped below the condensate take off point again and condensate removal can be re-continued. In the mean time more liquid may come in which can aggravate the problem, possibly even overfilling the slug catcher.
It is an object of the invention to provide an improved system for separating a fluid stream comprising gas, a light liquid and a heavy liquid which is more robust in handling liquid slugs.